You have to make sure that the lyrics are in one line as the "end-of-line" character is also the indication for "end-of-record".
So the next line of lyrics would be treated as the lyrics for the next file.
So: prepare your text file by replacing each "end-of-line" character with a character that is hardly ever used in normal language e.g. # or _.
E.g.
It's been a hard day's night
And I've been working like a dog
becomes
It's been a hard day's night_And I've been working like a dog
When importing the lyrics into the tag, enter %lyrics% as target field.
Actually, there are web sources plus tools that do some of the tricks for you. Check out
/t/2034/1